Heterojunction Bipolar transistors (HBT) are formed by utilizing different semiconductor materials for the emitter or collector and the base. The junction between layers of dissimilar materials being known as the heterojunction and the other junction between common semiconductor materials being a homojunction. In a single heterojunction bipolar transistor having a heterojunction between the emitter and the base, electrons are easily injected from the emitter to the base while hole injection from the base to the emitter is limited by a valence band offset between the emitter and base layers resulting in a current gain higher than a conventional homostructure device. FIG. 1 is an energy band diagram that shows the bandgap, the conduction band and valence band of a single heterojunction bipolar transistor in saturation. While the barrier between the emitter and base greatly reduces the possibility of recombination due to injection of holes back into the emitter region, there is an excess hole charge passing to the collector due to the lack of confinement in the junction between the base and the collector which adversely affects the gain of the transistor. An example of a single heterojunction bipolar transistor may be found in U.S. Pat. No. 4,593,305.
Recently, double heterojunction bipolar transistors (DHBT) have received significant attention. In order to solve the excessive hole charge problem with single HBTs, transistors were fabricated in which both the emitter and collector were formed of materials dissimilar to the base, thus forming a heterojunction at both the emitter/base and collector/base interfaces. FIG. 2 is an energy band diagram of a DHBT in saturation showing the presence of a bandgap in the valence band which includes abrupt barriers at both of the heterojunctions. The DHBT results in good hole confinement in the base region. However, the presence of the additional heterojunction at the base/collector junction also results in an abrupt barrier at this heterojunction in the conduction band which hinders the flow of electrons and hence current flow in the device. An example of the research on double heterojunction bipolar transistors can be found in PnP-type InP/InGaAsP/InP bipolar transistor, Electron Letters., Vol. 21, No. 21, October 10, 1985 by Su, et al. While Su, et al. show that a certain amount of grading at one of the heterojunctions can be obtained, there is still present a barrier to the flow of electrons prohibiting the realization of maximum gain and of minimum emitter-collector saturation voltage.